This invention relates to a control apparatus for a full-time four-wheel drive vehicle equipped with a center differential mechanism which permits a difference in rotating speed between the front and rear wheels caused by a difference in turning radius between the front and rear wheels produced when the vehicle is cornering.
The left and right wheels of an automotive vehicle generally have different turning radii at cornering. In order to compensate for the effects of this phenomenon and achieve cornering smoothly, a four-wheel drive vehicle is equipped with a differential mechanism (front and rear differentials) which absorbs the difference in rotating speeds between the left and right wheels in dependence upon the difference in turning radius. Since this phenomenon, namely the difference is turning radius, also develops between the front and rear wheels, it has been proposed to provide a four-wheel drive vehicle with a center differential mechanism which permits the difference in rotating speed between the front and rear wheels in dependence upon the difference in turning radius.
However, since the center differential mechanism functions to distribute front and rear wheel torque at an equal ratio, a limitation upon the transfer of driving force results in balance being achieved at either the front-wheel or rear-wheel driving force, whichever has the lower value. Consequently, there are situations where a four-wheel drive vehicle with a center differential mechanism exhibits a deterioration in transmitted driving force, as when there is a low coefficient of friction between the road surface and tires, in comparison with a four-wheel drive vehicle without a center differential mechanism. This can cause a phonomenon such as slipping (racing) of the front or rear wheels due to an inability to transfer the driving force to the road surface sufficiently, as when a large driving force is produced at acceleration.
In order to prevent these detrimental effects, a center differential locking mechanism is provided for a direct transfer of power between the front and rear wheels without the intervention of the center differential. When a large driving force becomes necessary, as during acceleration or when driving on a poor road surface, the center differential is locked manually. Under ordinary driving conditions when a large driving force is not required, the center differential is manually unlocked.
If the vehicle is traveling with the center differential mechanism locked, however, at cornering the front wheels must rotate faster than the rear wheels when the turning radius is small. Since the center differential mechanism is locked, however, the rotating speed differential between the front and rear wheels cannot be absorbed. As a result, a negative torque develops on the front-wheel side and it is just as if braking were being applied solely to the front wheels. In other words, a problem referred to as the "tight corner braking" phenomenon occurs. In this phenomenon, centrifugal force caused by the turn increases as the vehicle speed increases and the tires skid in the centrifugal direction. In consequence, the difference in rotating speed between the front and rear wheels is absorbed by the skidding of the tires. This has a deleterious influence upon the stability of the traveling vehicle during cornering. Manually unlocking the center differential mechanism is not a solution because this will not allow the condition of the road surface to be judged accurately and dealt with promptly.
Thus, if the center differential mechanism is locked during vehicle travel, the tight corner braking phenomenon occurs. In order to prevent this with assurance, it is necessary to monitor front wheel torque constantly and unlock the center differential mechanism automatically if, say, the front wheels develop a negative torque. This makes it necessary to measure the torque of the driven wheels. One method of measuring the torque of a driven wheel is to measure the angle of torsion of the drive shaft.
Rotary encoders have been proposed for the purpose of measuring a turning angle such as the torsional angle of a shaft. These encoders are of two types, namely an absolute-type and incremental type.
FIG. 9 illustrates a rotary encoder of the absolute type. The encoder includes a rotary shaft 1, a slitted disk 2 mounted on the rotary shaft 1, and a slitted disk 4 opposing the slitted disk 2. The disk 2 is provided with slits 3 in a plurality of concentric rows arranged from the circumference to the center of the disk, and the disk 4 is provided with slits 5 corresponding to each row of the slits 3. Light-emitting elements 6.sub.1 -6.sub.4 and light-receiving elements 7.sub.1 -7.sub.4 are arranged to oppose each other at positions corresponding to the rows of slits, with the disks 2, 4 being interposed therebetween.
Let "1" represent a state in which a light-receiving element is receiving light, and let "0" represent a state in which a light-receiving element is not receiving light. When the disk 2 rotates owing to rotation of the rotary shaft 1, the turning angle can be detected as a digital quantity from the combination of "0", "1" outputs from the light-receiving elements, this depending upon which of the light-receiving elements 7.sub.1 -7.sub.4 receive light.
FIG. 10 illustrates a rotary encoder of the incremental type. The encoder includes a rotary shaft 10, a slitted disk 11 mounted on the rotary shaft 10, and a slitted disk 13 opposing the slitted disk 11. The disk 11 is provided with slits 12 arranged in a single circumferentially extending row. The disk 13 is provided with slits 14, 15 staggered by one-fourth pitch with respect to the slits 12, and a slit 16 for producing a zero signal. Light-emitting elements 17.sub.1, 17.sub.2 and light-receiving elements 18.sub.1, 18.sub.2 are arranged to oppose each other with the disks 11, 13 interposed therebetween, and a light-emitting element 17.sub.3 and light-receiving element 18.sub.3, which are for producing the zero signal in cooperation with the slit 16, are arranged to oppose each other at positions corresponding to the slit 16, with the disks 11, 13 being interposed therebetween.
Whenever the disk 11 in the arrangement of FIG. 10 rotates by one pitch, namely through the angular spacing between neighboring ones of the slits 12, the light-receiving elements 18.sub.1, 18.sub.2 each produce an output pulse. The angle through which the disk 11 turns can be determined by counting these output pulses. Further, since the outputs of the light-receiving elements 18.sub.1, 18.sub.2 are out of phase by one-fourth cycle, the direction in which the disk 11 turns can be discriminated be detecting which output is leading or lagging in terms of phase.
However, the conventional rotary encoders of the types described above have slits which are very fine and complicated in shape, precision cannot be readily improved and costs are high owing to difficulties encountered in production techniques.